Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of communicating the signals over long distances with very low loss.
In telecommunications, information is often sent, received, and processed according to the Open System Interconnection Reference Model (OSI Reference Model or OSI Model). In its most basic form, the OSI Model divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers, which are also known respectively as Layer 7 (L7), Layer 6 (L6), Layer 5 (L5), Layer 4 (L4), Layer 3 (L3), Layer 2 (L2), and Layer 1 (L1). It is therefore often referred to as the OSI Seven Layer Model.
Layer 1 is the layer which typically defines electrical and physical specifications for devices. In particular, Layer 1 may define the relationship between a device and a transmission medium, such as a copper or optical cable. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more. In optical networks, Layer 1 may also be referred to as the “optical layer.”
Layer 2 is the layer which typically transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. Layer 2 provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the Layer 1. Examples of Layer 2 protocols are Ethernet for local area networks (multi-node), the Point-to-Point Protocol (PPP), HDLC and ADCCP for point-to-point (dual-node) connections. Layer 2 data transfer may be handled by devices known as switches.
Layer 3 is responsible for end-to-end (source to destination) packet delivery including routing through intermediate hosts, whereas Layer 2 is responsible for carrying Layer 3 packets of payloads and enabling communication between Layer 3 entities. Perhaps the best known example of a Layer 3 protocol is Internet Protocol (IP) and accordingly, Layer 3 is often referred to as the “IP layer.” Layer 3 data transfer may be handled by devices known as routers.
To ensure high reliability and availability in communications networks, including optical communications networks, redundancy is often built into such networks, so that if a fault occurs in a particular communications path, a redundant backup communication path may be utilized. In mesh networks, utilization of a backup path in response to a fault in a primary path may be referred to as “mesh restoration.” Network failures can originate from many different sources at multiple network layers. One major challenge in network restoration design is to develop a cost-effective network architecture that can restore failures at any network layer.
Although optical layer restoration has been extensively researched, and the technology for optical layer restoration is mature, pure IP layer restoration is currently preferred in many IP networks. One major reason for this preference is that the optical layer cannot restore failures at the IP layer; thus, additional IP layer capacity would still be needed for restoring IP layer failures, resulting in no cost advantage compared to pure IP layer restoration.